Primrose chrome yellow pigment and process for its preparation

ABSTRACT

A primrose type chrome yellow pigment of improved thermal stability is prepared. The pigment contains at least one hydrous oxide and an orthorhombic phase lead chromate-lead sulfate solid solution which is low in CO2 content. The solid solution is formed directly without precipitating a PbCO3 intermediate. The pigment may be encapsulated with a dense amorphous silica coating to achieve outstanding thermal stability characteristics.

United States Patent Jackson Dec. 2, 1975 PRIMROSE CHROME YELLOW PIGMENTAND PROCESS FOR ITS PREPARATION 3.370.971 2/1968 Linton 106/298 OTHERPUBLICATIONS Color Index, 2nd edition, 1956, published by the Society ofDyers and Colourists, England, and the American Association of TextileChemists and Colorists, United States.

Primary Examiner-Winston A. Douglas Assistant Examiner-John P. Sheehan[57] ABSTRACT A primrose type chrome yellow pigment of improved thermalstability is prepared. The pigment contains at least one hydrous oxideand an orthorhombic phase lead chromate-lead sulfate solid solutionwhich is low in CO content. The solid solution is formed directlywithout precipitating a PbCO intermediate. The pigment may beencapsulated with a dense amorphous silica coating to achieveoutstanding thermal stability characteristics.

4 Claims, N0 Drawings PRIMROSE CHROME YELLOW PIGMENT AND PROCESS FOR ITSPREPARATION BACKGROUND OF THE INVENTION In the manufacture of theso-called primrose type of chrome yellow (or lead chromate) basepigments, it has been conventional in the art that they be produced by aseries of steps wherein a precipitate of lead carbonate is initiallyformed and then converted to a stabilized orthorhombic solid solution oflead chromatelead sulfate by means of a highly acidic reaction medium.Although such pigments have been widely used in finishes, printing inksand the like, they have often tended to be deficient in thermalstability and hence less well suited for applications requiring a hightemperature processing step, e.g., as a colorant in plastic extrusions.While the chemical prereduction of lead chromate base pigments can serveto increase thermal stability as well as lightfastness, this is notaltogether an attractive solution since the pigment color intensity isdeleteriously affected.

One possible explanation for the poor thermal stability properties whichcharacterize certain of the primrose yellow pigments resides in thepresence of traces of CO as such or as a carbonate, but in either casedoubtlessly as a residue from the original lead carbonate precipitate,which is known to darken upon heating. Whatever the explanation be,however, an alternative procedure for the synthesis of such primroseyellow pigments would be highly desirable, particularly if attendanthigh level thermal stability characteristics were achieved without asignificant sacrifice to pigment color intensity.

SUMMARY OF THE INVENTION In accordance with the invention there isprovided a primrose type of chrome yellow base pigment characterized bya high level of thermal stability, particularly when encapsulated with adense silica coating. The pigment contains 0.5 to weight percent of oneor more hydrous oxides, preferably these being composed in part or inwhole of hydrous metal oxides, and 90 to 99.5 weight percent of anorthorhombic phase lead chromate-lead sulfate solid solution whereinthere is no more than about 0.35, preferably no more than 0.25, weightpercent of CO calculated as PbCO The low amount of CO (calculated asPbCO compares with commercial primrose pigments which can contain up toseveral percent of CO (calculated as PbCO seldom as little as even 0.75percent.

The primrose type or shade of lead chromate (or chrome yellow) pigmentsare referred to in Colour Index, 2nd edition, 1956, published jointly bythe Society of Dyers and Colourists, England, and the AmericanAssociation of Textile Chemists and Colorists, Unites States, under thedesignation Lead Sulfochromate CI-77603. They are represented by thegeneral formula PbCrO .x PbSO and usually contain 40 to 85 weightpercent of lead chromate and to 60 weight percent of PbSO, in solidsolution, the crystal phase of the particles being orthorhombic.

The pigments of the invention are characterized by a high level ofthermal stability. It is noteworthy that this is achieved without achemical reduction having occurred, i.e., without any chemical treatmentof the lead chromate-lead sulfate solid solution which would effect areduction to a lower valence state of the CrO; ions at the surface ofthe pigment particles. The absence of such a prereduction means thatthere has been no predarkening of the pigment, hence the highly intenseprimrose shade which characterizes the lead chromatelead sulfate solidsolution base is retained. While such pigments could be subsequentlysubjected to a chemical reduction, e.g., as in US. Pat. No. 2,808,339,to J. Jackson, this would necessarily involve a sacrifice in colorintensity.

The above-describedpigments of the invention may be prepared by aprocess that involves:

a. admixing a first aqueous solution containing a soluble lead salt andhaving a pH of 3.8 to 4.2 with a second aqueous solution having a pH of6.5 to 7 and containing chromate and sulfate ions, the admixing beingconducted in the presence of a combination of acetic acid, Al ions, andP O-f ions to precipitate the solid solutions, as above described, in astable orthorhombic crystal phase, the composition of the two solutionsbeing selected to produce the solid solution composition in the form ofa slurry having a pH of 1.4 to 2,

b. adjusting the pH of the aqueous slurry thus formed to 4 to 6 andmaintaining it at an elevated temperature of 40 to C. for at least 5minutes,

c. thereafter surface-coating the precipitate with 0.5 to 10 percent,based on the final pigment weight, of at least one hydrous oxide tofurther enhance lightfastness, and

d. .filtering the surface-coated precipitate, washing it free ofresidual ions, drying it, and then heating the pigment for a minimum of15 minutes to a temperature of at least C. to maximize lightfastness.

As regards the details of the above-described process, step (a) isadvantageously performed by admixing an aqueous lead nitrate solutionhaving a pH of about 3.8 to 4.2 with an aqueous solution containingsodium chromate, sodium dichromate: and sodium sulfate and having a pHof 6.5 to 7. The presence of small amounts of acetic acid, aluminum ionsand pyrophosphate ions is required in the latter solution to insureprecipitation of a solid solution in a stable orthorhombic crystalphase. The precipitation of the solid solution, upon mixing the twoaqueous solutions, results in a slurry having a pH of 1.4 to 2,preferably 1.4 to 1.8.

' According to step (b), the resultant aqueous slurry is adjusted to apH of4 to 6 and maintained at an elevated temperature of 40 to 80C. forat least 5 minutes to perfect the crystal structure of the solidsolution particles.

Subsequently, according to step (c), the precipitate is surface-coatedwith at least one hydrous oxide to further enhance lightfastness. Theamount of hydrous oxide so employed should be in the range of 0.5 to 10percent, based on the final pigment weight. The preferred oxide isalumina, but titania and zirconia can be utilized. Hydrous silica may beused but preferably only in conjunction with one of the aforementionedhydrous metal oxides, as this can be particularly beneficial in terms ofenhancing the lightfastness of the resultant pigment.

Finally, in step (d), the precipitate is filtered, washed and dried inthe conventional manner. It is essential, however, that in the course ofso recovering the pigment it be heated for a period of at least 15minutes at a temperature of at least 1 10C. to maximize lightfastness.Preferably this heating step will be conducted at a temperature of atleast C. for a period of one hour.

3 In any event failure to subject the pigment to this heating stepresults in a product of inferior qualities.

The novel chrome yellow pigments of the invention are particularlyuseful when coated with dense amorphous silica as described in LintonU.S. Pat. No. 3,370,971, i.e., to further improve resistance todiscoloration upon contact with acids, alkalies, and soap solutions, andupon exposure to light and heat. in the manufacture of such pigments thebase particles are treated so as to deposit on their surfaces, as asubstantially continuous coating, dense, amorphous silica followed by,optionally, the deposition of alumina on the silica. A typical processfor the production of such pigments involves slurrying the baseparticles in an aqueous medium and depositing amorphous silica on themfrom an aqueous sodium silicate solution at a pH above 6 and atemperature above 60C. Optionally, then, an aqueous solution of analuminum-containing compound may be added to the resulting suspension ofsilica-coated base particles with continued heating above 60C., wherebyalumina is deposited on the silica coating. Such coated pigments areparticularly advantageous in the plastics and paint industries.

Apart from the selection of the primrose chrome yellow base, othermaterials involved in the silica encapsulation process and theprocessing conditions to be employed therewith are described in theLinton patent, the disclosure of which is incorporated herein byreference.

The quantity of silica to be applied in the pigment treatment can bevaried over a considerable range depending on the intended end use ofthe pigment. In genera] the dense amorphous silica coating should be inthe range of about 2 to 40 percent by weight of the final pigment. Whenalumina is also present, a convenient quantity thereof is 0.25 to 2percent by weight of the final pigment although as much as percent canbe used without deleterious effect.

The products of the invention, particularly when silica-coated, are mostadvantageous when used in the formation of paints, printing inks,plastics and other products where outstanding thermal stability isdesired.

The following examples illustrate this invention in detail. Parts andpercentages as used therein are by weight unless otherwise stated.

EXAMPLE I A. A solution of 347 grams lead nitrate, Pb(NO in 2,500 cc. ofwater at 26.7C. is prepared and the pH of the solution is adjusted to4.0 with addition of nitric acid.

B. A second solution, at 267C, is prepared by dissolving in 1246 cc. ofwater containing 4 cc. of acetic acid:

28.5 grams Na Cr O .2H O

93 grams Na CrO 15.8 grams Na SO 1.7 grams Na P O The pH of thissolution is 6.7. To it is added, with stirring, grams Al (SO .18H Odissolved in 80 cc. water, and water dilution is effected to a totalvolume of 2,500 cc. The pH of solution B is then 6.45.

Solution B is added, with stirring, over a S-second period, to the leadnitrate solution A. The pH is found to be 1.5 to 1.6, and adjustmentupward to 4.3 is made by addition of 20 grams Na CO dissolved in 80 cc.of water.

The resulting slurry is heated to 54C. and held at this temperature for12 minutes, after which there are added in sequence with intermittent30-second stir periods, the following:

1. 8 grams Na CO in 32 cc. water;

2. a solution comprising: 44.4 grams Na siO (Du Pont 20WW Grade) 2.4grams 50% NaOH made up to 300 cc. with water (The pH of the slurry isfound to be 8.5 to 8.6 at this point);

3. 200 cc. of an aqueous solution containing 1.0 gram MnSO .H O;

4. 40 grams of Al (SO .18H O in cc. of water;

and

5. 13 grams Na CO to adjust the pH of the slurry to The precipitate isrecovered by filtration and waterwashed SO -free, dried at 80C. andbaked 2 hours at C. The CO content of the pigment is about 0.18 percentby weight, calculated as PbCO The pigment is then silica-coated by thegeneral procedure of Linton US. Pat. No. 3,370,971. Thus grams of thepigment are slurried in 1,000 cc. water containing 20 grams sodiumsilicate (Du Pont 20 WW Grade). The slurry is passed through aManton-Gaulin homogenizer at 5,000 psig. The homogenizer is washed outwith 100 cc. water and the pH is 10.5.

The homogenized slurry is heated to 90C. and the pH adjusted to 8.0.This temperature is maintained while the following treatment steps arecarried out:

Over a 4-hour period, at a uniform rate, there is added a solution of l15 grams Na SiO (Du Pont 20WW Grade) in 600 cc. water. Simultaneously,but over a 5-hour period, there is added, at a uniform rate, a solutionof 17.3 grams of 98% H SO in 800 cc. water. The pH of the treated slurryis found to be 5.8, and this is adjusted to 9.0 by addition of 5% NaOHsolution. The pigment is then coagulated by addition of two solutionswith an intervening two-minute stirring period of:

Solution 1: K Wood Rosin 20 grams NaOH 2.7 grams in ml H O (Mixtureheated to the boil to dissolve) Solution 2: CaCl .2H O 9 grams in 50 m1H O.

The slurry is stirred for 5 minutes, filtered, washed and dried at 90C.,and micropulverized.

A silica-coated pigment labeled as I and produced by the generalprocedure given above is tested for heat stability in a polystyrenemolding versus a commercial prereduced primrose chrome yellow pigment,also coated with silica in the manner given above, as a Control. Thechrome yellow base of the Control pigment contains antimony as aprereducing metal and contains CO in amounts of about 0.8 to 2.4percent, calculated as PbCO (depending upon the particular lotemployed). The pigments are compounded at the 1 percent level withpolystyrene and portions molded at 400F. and 600F. Spectrophotometriccurves of the two materials are made to show the color intensity ofeach.

The Sample 1 pigment is clearly superior in thermal stability as colorintensity changes upon heating to less of an extent than does theControl. The superiority is apparent at 400F. but is even morepronounced at 600F. It is noted that the superiority is demonstrated inspite of the fact that the Control sample had been chemically prereducedto improve its thermal stability.

sentially of 40 to 85 weight percent of lead chrornatc EXAMPLE [I and to60 weight percent of lead sulfate, said solid In this case thepreparation of the primrose base is solution containing no more thanabout 0.35 weight the same as given in Example 1 except that solution13" percent of CO calculated as i 'bCO by: is prepared as follows: 5 a.admixing a first aqueous solution containing a SOlU-- ble lead salt andhaving a pH of 3.8 to 4.2 with a Time isdiswived in 1200 m H2O secondaqueous solution having a pH of 6.5 to 7 mi s%,t,-isti,o griih sdissolved in 80 cc. of water an.d.conta.mmg chromate. and Sulfate lcmsthe ad- NLHPZCL L7 grams dissolved in 200 CC mixing being conducted inthe presence of a comg of water bination of acetic acid, Al ions, and P0 ions iff; Acct to precipitate the solid solution as a stable ortho-Ntt,Cr,0,-2H,O 28.5 grams rhombic crystal phase, the composition of thetwo z i 93 grams solutions being selected to produce the solid solu 15tion composition in the form of a slurry having a The above is thendiluted to 2,500 ml, and a pH of 6.6 P of ll0 is re orded b. adjustingthe pH of the aqueous slurry thus formed This base pigment is alsosilica-coated by the general to 4 to 6 and maintaining it at an elevatedprocedure of Example I to produce a pigment also ture of to for 29163915minumsi rated excellent for th l bilit c. thereafter surface-coating theprecipitate with 0.5 to 10 percent, based on the final pigment weight ofEXAMPLE at least one hydrous oxide selected from the group in this casethe preparation of the primrose base is Consisting of alumina, mania andZlTCOnia to also the same as given in Example 1 but in step (5) thethfir enhance lightfaStneSS, and pH is adjusted to 4.9 in tead of 5,4 timprove fil d. filtering the surfacecoated precipitate, washing itability. The coating of the base pigment with dense silfree of residualions, drying it and then heating the ica is as follows: pigment for aminimum of 15 minutes to a temper- A 150 gram portion of the pigment islu -ied i 450 ature of at least l 10C. to maximize lightfastness. cc.water containing 20 grams sodium silicate (Du Pont A Chr mfi yellopigment 0f the primrose type No. 9 Grade). The slurry is a ed through aMantonhaving excellent color intensity and improved thermal Gaulinhomogenizer at 2,000 psig., the homogenizer stability characteristics,said pigment consisting essenwashed out with 150 cc. water, a secondpass is then tially of 0.5 to 10 weight percent of one or more hymadethrough the homogenizer at 5,000 psig, and a drous oxides, selected fromthe group consisting of aluwashing then made with 640 cc. water. The pHof final mina, titania and zirconia, and 90 to 95.5 weight perslurry is10.5. cent of an orthorhombic phase lead chromate-lead sul- Thehomogenized slurry is thereafter heated to 90C. fate solid solutionconsisting essentially of to 85 and the pH adjusted to 9.0. Thistemperature is mainweight percent of lead chromate and 15 to 60 weighttained while the following treatment steps are carried percent of leadsulfate, said solid solution containing no out: more than about 0.35weight percent of CO calcu-- Over a 3-hour period, at a uniform rate,there is 40 lated as PbCO added a solution of l 15 grams Na SiO (Du PontNo. 9 3. A chrome yellow pigment according to claim 2 Grade) in 540 cc.water. Simultaneously, but over a 3 wherein the hydrous oxide isalumina. %-h0l11' period, there is added, at a uniform rate, a solu- 4.A chrome yellow pigment of the primrose type tion of 17.3 grams of 98% H30 in 735 cc. water. To having excellent color intensity and improvedthermal the slurry there is added 1 1 grams alum dissolved in 50stability characteristics, said pigment consisting essen cc. water,giving a pH of 3.5 to 3.8. Then the pH is adtially of 90 to 99.5 weightpercent of an orthorhombic justed to 4.2 to 4.5 with 5% NaOH solution.Finally the phase lead c'hromate-lead sulfate solid solutionconsistslurry is stirred for 5 minutes, filtered, washed and ingessentially of 40 to 85 weight percent of lead chrodried at 104C, andrnicropulverizecl. mate and 15 to 60 weight percent of lead sulfate,said Again the pigment is rated excellent in terms of thersolid solutioncontaining no more than about 0.35 mal stability. weight percent of COcalculated as PbCO and 0.5 to

What is claimed is: 10 weight percent ofa surface-coating consistingessenll. A process for the production of a primrose yellow tially of oneor more hydrous oxides selected from the shade pigment consistingessentially of 0.5 to 10 weight group consisting of alumina, titania andzirconia, said percent of one or more hydrous oxides selected frompigment being encapsulated with about 2 to 40 weight the groupconsisting of alumina, titania and zirconia, percent, based on theweight of the encapsulated pig-- and 90 to 99.5 weight percent of anorthrhombic phase ment, of dense amorphous silica. lead chromate-leadsulfate solid solution consisting es-

1. A PROCESS FOR THE PRODUCTION OF A PRIMROSE YELLOW SHADE PIGMENTCONSISTING ESSENTIALLY OF 0.5 TO 10 WEIGHT PERCENT OF ONE OR MOREHYDROUS OXIDES SELECTED FROM THE GROUP CONSISTING OF ALUMINA, TITANIAAND ZIRCONIA AND 90 TO 99.5 WEIGHT PERCENT OF AN ORTHRHOMBIC PHASE LEADCHROMATE-LEAD SULFATE SOLID SOLUTION CONSISTING ESSENTIALLY OF 40 TO 85WEIGHT PERCENT OF LEAD CHROMATE ANND 15 TO 60 WEIGHT PERCENT OF LEADSULFATE, SAID SOLID SOLUTION CONTAINING NO MORE THAN ABOUT 0.35 WEIGHTPERCENT OF CO2, CALCULATED AS PBCO3 BY: A. ADMIXING A FIRST AQUEOUSSOLUTION CONTAINING A SOLUBLE LEAD SALT AND HAVING A PH OF 3.8 TO 4.2WITH A SECOND AQUEOUS SOLUTION HAVING A PH OF 6.5 TO 7 AND CONTAININGCHROMATE AND SULFATE IONS, THE ADMIXING BEING CONDUCTED IN THE PRESENCEOF A COMBINATION OF ACETIC ACID, AI+3 IONS, AND P2O7-4 IONS TOPRECIPITATE THE SOLID SOLUTION AS A STABLE ORTHORHOMBIC CRYSTAL PHASE,THE COMPOSITION OF THE TWO SOLUTIONS BEING SELECTED TO PRODUCE THE SOLIDSOLUTION COMPOSITION IN THE FORM OF A SLURRY HAVING A PH OF 1.4 TO 2, B.ADJUSTING THE PH OF THE AQUEOUS SLURRY THUS FORMED TO 4 TO 6 ANDMAINTAINING IT AT AN AN ELVATED TEMPERATURE OF 40* TO 80*C. FOR AT LEAST5 MINUTES, C. THEREAFTER SURFACE-COATING THE PRECIPITATE WITH 0.5 TO 10PERCENT, BASED ON THE FINAL PIGMENT WEIGHT OF AT LEAST ONE HYDROUS OXIDESELECTED FROM THE GROUP CONSISTING OF ALUMINA, TITANIA AND ZIRCONIA TOFURTHER ENHANCE LIGHT-FASTNESS, AND D. FILTERING THE SURFACE-COATEDPRECIPITATE, WASHING IT FREE O F RESIDUAL IONS, DRYING IT AND THENHEATING THE PIGMENT FOR A MINIMUM OF 15 MINUTES TO A TEMPERATURE OF ATLEAST 110*C. TO MAXIMIZE LIGHTFASTNESS.
 2. A CHROME YELLOW PIGMENT OFTHE PRIMROSE TYPE HAVING EXCELLENT COLOR INTENSITY AND IMPROVEMENTTHERMAL STABILITY CHARACTERISTICS, SAID PIGMENT CONSISTING ESENTIALLY OF0.5 TO 10 WEIGHT PERCENT OF ONE OR MORE HYDROUS OXIDES, SELECTED FROMTHE GROUP CONSISTING OF ALUMINA, TITANIA AND ZIRCONIA AND 90 TO 95.5WEIGHT PERCENT OF AN ORTHORHOMBIC PHASE LEAD CHROMATE-LEAD SULFATE SOLIDSOLUTION CONSISTING ESSENTIALLY OF 40 TO 85 WEIGHT PERCENT OF LEADCHROMATE AND 15 TO 60 WEIGHT PERCENT OF LEAD SULFATE, SAID SOLIDSOLUTION CONTAINING NO MORE THAN ABOUT 0.35 WEIGHT PERCENT OF CO2,CALCULATED AS PBCO3.
 3. A chrome yellow pigment according to claim 2wherein the hydrous oxide is alumina.
 4. A chrome yellow pigment of theprimrose type having excellent color intensity and improved thermalstability characteristics, said pigment consisting essentially of 90 to99.5 weight percent of an orthorhombic phase lead chromate-lead sulfatesolid solution consisting essentially of 40 to 85 weight percent of leadchromate and 15 to 60 weight percent of lead sulfate, said solidsolution containing no more than about 0.35 weight percent of CO2,calculated as PbCO3, and 0.5 to 10 weight percent of a surface-coatingconsisting essentially of one or more hydrous oxides selected from thegroup consisting of alumina, titania and zirconia, said pigment beingencapsulated with about 2 to 40 weight percent, based on the weight ofthe encapsulated pigment, of dense amorphous silica.